Ligand-Free RuCl3-Catalyzed Alkylation of Methylazaarenes with Alcohols

Modular assembly of arenes, ethylene and heteroarenes for the synthesis of 1,2-arylheteroaryl ethanes

The 1,2-arylheteroaryl ethane motif stands as a privileged scaffold with promising implications in drug discovery. Conventional de novo syntheses of these molecules have relied heavily on pre-functionalized synthons, entailing harsh conditions and multi-step processes. Here, to address these limitations, we present a modular approach for the direct synthesis of 1,2-arylheteroaryl ethanes using feedstock chemicals, including ethylene, arenes and heteroarenes. We disclosed a photo triplet-energy-transfer-initiated radical cascade process, leveraging homolytic cleavage of C-S bonds in aryl sulfonium salts as the key step to access aryl radicals with excellent regioselectivity. This method allows for rapid structural diversification of bioactive molecules, showcasing excellent functional group tolerance and streamlining the synthesis of bioactive compounds and their derivatives. Furthermore, our approach can be extended to propylene, non-gaseous terminal alkenes and various other electrophilic radical precursors, including heteroaryl radicals, hydroxyl radicals, trifluoromethyl radicals and α-carbonyl alkyl radicals. This study highlights the significance of radical polarity matching in designing selective multi-component couplings.

Transfer Hydrogenation of N- and O-Containing Heterocycles Including Pyridines with H3N-BH3 Under the Catalysis of the Homogeneous Ruthenium Precatalyst

In this study, we report a transfer hydrogenation protocol that utilizes borane-ammonia (H3N-BH3) as the hydrogen source and a commercially available RuCl3·xH2O precatalyst for the selective aromatic reduction of quinolines, quinoxalines, pyridines, pyrazines, indoles, benzofurans, and furan derivatives to form the corresponding alicyclic heterocycles in good to excellent isolated yields. Applications of this straightforward protocol include the efficient preparation of useful key pharmaceutical intermediates, such as donepezil and flumequine, including a biologically active compound.

First-Row Transition-Metal Catalyzed Acceptorless Dehydrogenation and Related Reactions: A Personal Account

The development of sustainable catalytic protocols that circumvent the use of expensive and precious metal catalysts and avoid toxic reagents plays a crucial role in organic synthesis. Indeed, the direct employment of simple and abundantly available feedstock chemicals as the starting materials broadens their synthetic application in contemporary research. In particular, the transition metal-catalyzed diversification of alcohols with various nucleophilic partners to construct a wide range of building blocks is a powerful and highly desirable methodology. Moreover, the replacement of precious metal catalysts by non-precious and less toxic metals for selective transformations is one of the main goals and has been paid significant attention to in modern chemistry. In view of this, the first-row transition metal catalysts find extensive applications in various synthetic transformations such as catalytic hydrogenation, dehydrogenation, and related reactions. Herein, we have disclosed our recent developments on the base-metal catalysis such as Mn, Fe, Co, and Ni for the acceptorless dehydrogenation reactions and its application in the C-C and C-N bond formation via hydrogen auto-transfer (HA) and acceptorless dehydrogenation coupling (ADC) reactions. These HA/ADC protocols employ alcohol as alkylating agents and eliminate water and/or hydrogen gas as by-products, representing highly atom-efficient and environmentally benign reactions. Furthermore, diverse simple to complex organic molecules synthesis by C-C and C-N bond formation using feedstock alcohols are also overviewed. Overall, this account deals with the contribution and development of efficient and novel homogeneous as well as heterogeneous base-metal catalysts for sustainable chemical synthesis.

Manganese catalyzed C-alkylation of methyl N-heteroarenes with primary alcohols

C-Alkylations of nine different classes of methyl-substituted N-heteroarenes are disclosed using a bench stable Mn(i)-catalyst under borrowing hydrogen conditions.

Nickel-catalyzed C3-alkylation of indoles with alcohols via a borrowing hydrogen strategy

An efficient Ni-catalyzed C3-alkylation of indoles with alcohols via a borrowing hydrogen pathway was achieved utilizing an N,O-donor coordinated nickel complex as the precatalyst.

An alternative mode to activate alcohols: application to the synthesis of N-heteroarene derivatives

Activation of primary alcohols in the presence of KOtBu/DMF allowed the synthesis of N-heteroarenes via the alkylation of the C(sp3)–H bond of methyl azaarenes. A mechanism involving the formation of an alkyl formate intermediate is proposed.

Green Approach for Visible-Light-Induced Direct Functionalization of 2-Methylquinolines

A transition metal- and oxidant-free visible light-photoinduced protocol for direct functionalization of 2-methylquinolines has been developed. This protocol enabled the C-H functionalization of substituted 2-methylquinolines with diacetyl or ethyl pyruvate, under environmentally friendly conditions. A mechanistic investigation based on density functional theory (DFT) calculations provided details about the origins of reactivity and selectivity.

Iron-catalysed alkylation of 2-methyl and 4-methyl azaarenes with alcohols via C-H bond activation

The first Fe-catalysed alkylation of 2-methyl and 4-methyl-azaarenes with a series of alkyl and hetero-aryl alcohols is reported (>39 examples and up to 95% yield). Multi-functionalisation of pyrazines and synthesis of anti-malarial drug (±) Angustureine significantly broaden the scope of this methodology. Preliminary mechanistic investigation, deuterium labeling and kinetic experiments including trapping of the enamine intermediate 1a' are of special importance.

Ligand-Free Ru-Catalyzed Direct sp3 C-H Alkylation of Fluorene Using Alcohols

The sp³ C-H alkylation of 9H-fluorene using alcohol and a Ru catalyst via the borrowing hydrogen concept has been described. This reaction was catalyzed by the [Ru(p-cymene)Cl₂]₂ complex (3 mol %) and exhibited a broad reaction scope with different alcohols, allowing primary and secondary alcohols to be employed as nonhazardous and greener alkylating agents with the formation of environmentally benign water as a byproduct. A variety of 9H-fluorene underwent selective and exclusive mono-C9-alkylation with primary alcohols in good to excellent isolated yield (26 examples, 50-92% yield), whereas this reaction with secondary alcohols in the absence of any external oxidants furnished the tetrasubstituted alkene as the major product. Furthermore, a base-mediated C-H hydroxylation of the synthesized 9H-fluorene derivatives afforded 9H-hydroxy-functionalized quaternary fluorene derivatives in excellent yield.

Recent trends in catalytic sp3 C-H functionalization of heterocycles

Heterocycles are a ubiquitous substructure in organic small molecules designed for use in materials and medicines. Recent work in catalysis has focused on enabling access to new heterocycle structures by sp³ C-H functionalization on alkyl side-chain substituents-especially at the heterobenzylic position-with more than two hundred manuscripts published just within the last ten years. Rather than describing in detail each of these reports, in this mini-review we attempt to highlight gaps in existing techniques. A semi-quantitative overview of ongoing work strongly suggests that several specific heterocycle types and bond formations outside of C-C, C-N, and C-O have been almost completely overlooked.

Cobalt-catalyzed alkylation of methyl-substituted N-heteroarenes with primary alcohols: direct access to functionalized N-heteroaromatics

Phosphine free, air and moisture stable Co(NNN) complex catalyzed alkylation of various methyl-substituted N-heteroarenes with alcohols is reported. Following the borrowing hydrogen methodology, a variety of methyl-substituted N-heteroarenes can be functionalized efficiently. To understand the mechanism of this reaction various kinetic and control experiments were carried out.

Reaction condition controlled nickel(ii)-catalyzed C-C cross-coupling of alcohols

The challenge in the C-C cross-coupling of secondary and primary alcohols using acceptorless dehydrogenation coupling (ADC) is the difficulty in accurately controlling product selectivities. Herein, we report a controlled approach to a diverse range of β-alkylated secondary alcohols, α-alkylated ketones and α,β-unsaturated ketones using the ADC methodology employing a Ni(ii) 4,6-dimethylpyrimidine-2-thiolate cluster catalyst under different reaction conditions. This catalyst could tolerate a wide range of substrates and exhibited a high activity for the annulation reaction of secondary alcohols with 2-aminobenzyl alcohols to yield quinolines. This work is an example of precise chemoselectivity control by careful choice of reaction conditions.

Ni-Catalyzed α-Alkylation of Unactivated Amides and Esters with Alcohols by Hydrogen Auto-Transfer Strategy

A transition-metal-catalyzed borrowing hydrogen/hydrogen auto-transfer strategy allows the utilization of feedstock alcohols as an alkylating partner, which avoids the formation of stoichiometric salt waste and enables a direct and benign approach for the construction of C-N and C-C bonds. In this study, a nickel-catalyzed α-alkylation of unactivated amides and ester (tert-butyl acetate) is carried out by using primary alcohols under mild conditions. This C-C bond-forming reaction is catalyzed by a new, molecularly defined nickel(II) NNN-pincer complex (0.1-1 mol %) and proceeds through hydrogen auto-transfer, thereby releasing water as the sole byproduct. In addition, N-alkylation of cyclic amides under Ni-catalytic conditions is demonstrated.

Stable Fe(ii)-based coordination polymers: synthesis, structural diversity and catalytic applications in homo-coupling reactions

Herein, we synthesize three new stable Fe(ii) coordination polymers, {[Fe3(tttmb)4(OH)(NCS)5]2CH3OH·H2O}n (1), {[Fe3(tttmb)4(NCS)6]·3H2O}n (2), and {[Fe3(tttmb)2Cl6(H2O)6]·5H2O}n (3) (tttmb = 1,3,5-tris(triazole-1-ylmethyl)-2,4,6-trimethyl-benzene) under the regulation of KSCN and solvents and use them as green heterogeneous catalysts in aryl homo-coupling reactions. The catalytic experiments show that 1 and 3 exhibit high efficiency for aryl homo-coupling reactions under an air environment with a yield of up to 80%. Furthermore, the crystal structure analysis reveals that 1 and 3 have coordinated water molecules and OH-, which are easily removed from the host materials during the reactions. In contrast, no unsaturated coordination sites are generated in 2 during the reaction process. In addition, the variable temperature magnetic susceptibilities of 1-3 prove that the metal centers of these polymers are bivalent and all of them exhibit a weak antiferromagnetic effect.

Ni-Catalyzed dehydrogenative coupling of primary and secondary alcohols with methyl-N-heteroaromatics

Here we report the first base-metal catalyzed dehydrogenative coupling of primary (aromatic, heteroaromatic, and aliphatic) and secondary alcohols with methyl-N-heteroaromatics to form various C(sp³)-alkylated N-heteroaromatics.

Effects of water on a catalytic system for preparation of N-(1,4-dimethylamyl)-N′-phenyl-p-phenylenediamine by reductive alkylation

Effects of water on a catalytic reaction system for reductive alkylation of p-aminodiphenylamine (p-ADPA) with 5-methyl-2-hexanone (MIAK) was studied. Platinum nanoparticles supported on activated carbon with high specific surface area were used as catalysts for reductive alkylation reaction under different water content conditions. Schiff base forming data, catalytic activity and stability of the catalysts were investigated under the aforementioned reaction conditions. Fresh and used catalysts were characterized by TEM, SEM, XRD, ICP, laser particle size analysis, N₂ physical adsorption and CO chemical adsorption to explore the effects of water on the catalytic reaction system for the ketone/amine reductive alkylation reaction. The characterization results indicated that catalyst support pulverization and Pt loss occurred in the reused catalyst, and the trend was more obvious under the conditions of higher water content. Water in the reaction system could also aggravate the decrease of the catalyst's specific surface area and pore volume, which should be a major reason of the lower catalytic performance.

Water could restrain the formation of a Schiff base, reducing the selectivity of 7PPD and aggravating the decrease of catalytic performance.

Nickel-catalysed alkylation of C(sp3)–H bonds with alcohols: direct access to functionalised N-heteroaromatics

Base-metal catalysed C(sp³)–H bond functionalisation of methyl-N-heteroaromatics with alcohols is reported for the first time.